The present invention relates generally to circuits of the type in which the output signal of multivibrator apparatus is controlled by a variable capacitance sensor element, and more particularly to such a circuit in which the capacitance value of the sensor element determines the width of a pulse produced at a repetition rate independent of the capacitance value.
It is well known that the capacitance of an element may be affected by certain environmental parameters, e.g., humidity. This characteristic has long been used as a basis for a variety of sensors and sensing systems. Such a sensor or system typically includes a capacitive sensor element specifically designed and constructed to enhance its response to the parameter of interest, and an electrical circuit, such as a bridge network, responsive to variations in capacitance of the sensor element.
Another approach to generating a sensed parameter signal from the capacitance of a variable capacitance sensor element is shown in the following documents:
(1) VALVO report TI 790423, "Sensor zur Messung der relativen Luftfeuchte", Philips GmbH, 11 pages
(2) Technical note 134, "Capacitive humidity sensor for consumer applications", Philips, 7 pages
(3) Technical information 063, "Capacitive humidity sensor", 15 pages (20 Nov. 1980)
The circuit shown and described in these documents basically uses two multivibrators, of which one generates a rectangular wave whose characteristics are proportional to the capacitance of a reference capacitor. This multivibrator is coupled to the second multivibrator in such a way that a rectangular wave produced by the second multivibrator is synchronized with that produced by the first multivibrator, the rectangular wave generated by the second multivibrator having a pulse duration which proportional to the capacitance of a sensing capacitor. The rectangular wave outputs of both multivibrators are logically combined to produce a pulse train in which the pulse duration corresponds to the difference in the pulse durations of the signals produced by the two multivibrators.
Yet another approach to utilizing the capacitance of a sensing capacitor for indicating an environmental parameter is shown in U.S. Pat. No. 4,563,634 issued to E. Lehle on Jan. 7, 1986. This approach involves a single multivibrator having cross coupled branches, one of which includes a capacitive sensor having a capacitance C.sub.sens, and the other of which includes a reference capacitor having a capacitance C.sub.ref. An output signal taken from each of these branches is averaged by a separate resistor-capacitor network, the average values being supplied to opposing inputs of a differential amplifier. The differential amplifier provides an output signal whose value, U.sub.A, is given by the following equation: ##EQU1## The output signal of the differential amplifier is also used to vary the voltage supplied to the reference and sensing capacitors to partially compensate for the non-linear response which is inherent in capacitor based sensors and sensing systems. However, this compensation does not completely linearize the circuit response. In addition, the described circuit arrangement precludes use of the differential amplifier for providing independent offset compensation since both input terminals of the differential amplifier are required for receiving the two averaged output signals from the multivibrator.